The present invention relates generally to a light source unit, and more particularly to a light source for an exposure apparatus that produces fine patterns for micromechanics and manufactures various devices, e.g., a semiconductor chip, such as an IC and an LSI, a display device, such as a liquid crystal panel, a sensing device, such as a magnetic head, and an image-pickup device, such as a CCD. The inventive light source unit is suitable, for example, for an exposure apparatus that uses an X-ray and extreme ultraviolet (“EUV”) light for a light source.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask (or a reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of the light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with the recent demands for finer processing to the semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet (“UV”) light has the limits to satisfy the rapidly promoting fine processing to the semiconductor device, and a reduction projection optical system using the EUV light with a wavelength of 10 to 15 nm shorter than that of the UV light has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The EUV light source uses, for example, a laser plasma light source. It irradiates a highly intensified pulse laser beam to a target material, such as a metallic thin film, inert gas and liquid droplets, in a vacuum chamber to generate high-temperature plasma for use as the EUV light with a wavelength of about 13 nm emitted from this.
As discussed, the EUV light source attracts attentions as a light source for the EUV exposure apparatus that manufactures the semiconductor devices, and various adjustments have been proposed. For example, one proposal detects the EUV light generated from (a generating position of) the plasma using a pinhole camera and a CCD, controls a target supplying position or a pulsed-laser irradiating position (which is a condensing position of the pulsed later), and maintains a generating position of the EUV light in place. See, for example, Japanese Patent Applications, Publication Nos. 2000-56099, 2000-340395, 2001-267096 and 2001-32096.
The prior art can adjust the light source when the generating position of the EUV light fluctuates in the EUV light source due to changes of a divergent angle and exit direction of the pulsed laser beam and a replacement of a target as a result of temperature changes of the laser rod and the crystal for generating second harmonics. However, the prior art cannot adjust the light source, for example, when a position and angle of a condenser mirror fluctuate and a positional relationship between the EUV light source and the subsequent optical system fluctuates due to the temperature changes and resonance. As a consequence, the EUV light supplied from the EUV light source does not become steady, and the exposure apparatus causes insufficient and non-uniform exposure dose, changes of an imaging position, and lowered exposure performance.